Author Topic: speed variation of a motor ?  (Read 2774 times)

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Offline harrysmith

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speed variation of a motor ?
« on: January 08, 2022, 05:34:56 pm »
Hi everyone, what is the principle of variation of the speed of an asynchronous motor ?

Offline JeffK

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Re: speed variation of a motor ?
« Reply #1 on: January 09, 2022, 09:15:46 pm »
I would suggest adding some more detail regarding your specific question. Unfortunately it is ambiguous. If you are asking as you don't know where to begin, that is perfectly acceptable. And would be very helpful information when replying to your inquiry. 

As you included the phrase "asynchronous motor" in your post, I'm guessing your question is specific to commercially available AC induction electric motors. One of the better resources to learn how they operate and what the information on the nameplate is all about is presented in a well known document in the industry. You can search for "The Cowern Papers". ABB has the document posted here:

Once you know a bit more about the operation of electric motors and some of the terminology, then you could add some more detail on what is meant by "speed variation". A key specification will be the time frame you are expecting to measure a change in speed. Milliseconds, Hours, Years? Another important specification will be the change in load on the motor. And finally what size motor is of interest?     

Offline TimFox

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Re: speed variation of a motor ?
« Reply #2 on: January 09, 2022, 10:24:45 pm »
If, by "asynchronous motor" you mean an AC induction motor, then the important detail is that the rotor lags the AC frequency (e.g., 1750 rpm for 60 Hz drive, instead of 1800 rpm).
The amount of "slip" depends on the torque:  as the load torque increases, the rotation speed falls.
Therefore, you can change the speed by changing the AC frequency, as in a variable-frequency drive, but the actual speed is not directly determined by the frequency only.
See the basic description:
On the right side of this article, there are several curves showing speed (horizontal axis) vs. load torque (vertical axis).  At zero load torque, the speed is "synchronous" (i.e., 1800 rpm for the example above), and the speed falls off as the torque increases (right side of the graph).  Ordinarily, motor operation is on that part of the curve on the right where the speed is a monotonic function of torque.

Offline geggi1

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Re: speed variation of a motor ?
« Reply #3 on: January 10, 2022, 06:10:34 am »
The most common way to regulate the speed of an AC induction motor (syncronus or asyncronus) is with avariable frequency drive.
If there is a requirement for high precision RPM is is possible to fit the motor with a puls decoder. The puls decoder can be as simple as a proximity sensor.
One thing that is important for motors controlled by a VFD is the requirement for external colling because the fan on the motor will not ventilate enought to cool the motor down. This is often done by an externel fan motor.
It is also possible to run a VFD from a singlephase supply (two phases) but only at 1/3 of the nominal load bacause the rectifier is not desigend for the full load on just two phases. There are some of these drives that is desigend for single phase operation, but these are special products.

Offline Siwastaja

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Re: speed variation of a motor ?
« Reply #4 on: January 10, 2022, 07:17:52 pm »
Assuming asynchronous motor = ac induction motor.

Principle is, varying the electrical frequency and amplitude by using variable frequency drive (VFD), now easily available as low-cost off-the-shelf unit.

On asynchronous motor, frequency control alone doesn't give good accuracy. Efficiency of simple V/f control isn't great either. This is why good VFDs control torque with more sophisticated algorithms, so what you really want is feedback: actual speed measurement (using optical encoder, magnetic sensor, whatever) and a control loop that outputs a torque setpoint for the VFD module.

Many VFDs already offer this whole shebang integrated for you, ready to use.

Offline rpiloverbd

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Re: speed variation of a motor ?
« Reply #5 on: February 14, 2022, 03:17:27 pm »
I think it's better to look at 'Electrical Machines' by BL Theraja for more details. 

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